Caption: Viagra. Computer artwork of a blue Viagra pill on a red and black background. Viagra is the trade- name for the drug sildenafil citrate, which is used to treat male impotence. It is the first oral pill for impotence. It does not cause penile erections directly, but instead affects the response to sexual stimulation. It enhances the smooth muscle relaxant effects of nitric oxide, a substance normally released in response to sexual stimulation. Smooth muscle relaxation allows blood to enter the penis, leading to an erection. Viagra is made by Pfizer Pharmaceuticals.